When running a bare "hg update" without specifying a revision, we used to
update to the tip of the current branch, even if it were bookmarked. This is
inconvenient when using bookmarks for feature branches, since pulling new
bookmarks will make "hg update" update to those bookmarked "feature branches".

This patch changes this behavior. "hg update" will look for the tipmost head of
the current branch that is not bookmarked. If none is found the old behavior is
used (i.e. we will update to the tip of the current branch).

This is missing tests and possibly a comment on the update command help.